1. Field of the Invention
The present invention relates to the field of optical storage media, and in particular to a method for producing an optical information carrier having a variable relief structure representing read-only data.
2. Description of Prior Art
Optical storage media, in particular optical disks, are a promising form of static and dynamic data storage. Optical disks containing read-only data, otherwise known as CD ROM disks, are especially well-suited for high-density permanent storage. These read-only devices are used for preserving large amounts of information, such as audio and video recordings, reference materials, etc. Currently there exists a need for optical data carriers having surface features with more than one displacement from the surface plane. A typical CD-ROM disk, on the other hand, has only one such displacement, which is typically 1/4 wavelength, .lambda.g, of the reading light used as measured in the medium of the disk's protective coat. Certain physical limitations under this design exist which vitiate the reliability of tracking as well as the packing density of data-bearing pits. In addition, the pits themselves have only one possible depth, in effect allowing only 1 bit of data to be represented per unit length of pit.
Proposed technologies exist in the art both for alleviating the tracking reliability problem and increasing the data storage density. Tracking solutions include the use of a guide groove of a different optical depth than the data bearing pits, e.g., as described by Abe in U.S. Pat. No. 5,144,552. Data storage capacity may be improved by either the use of a multi-depth data pit, which increases the number of bits per unit of pit length, as described by Bearden in U.S. Pat. Nos. 5,029,023 and 5,235,587, or by reducing the track-to-track spacing. In the latter case, alternate tracks of data pits are embossed with different optical depths so as to reduce the crosstalk from adjacent tracks and allow reliable reading. Techniques for forming such data pits are described in prior art, e.g., by Basilico in U.S. Pat. No. 4,144,752, Setani in U.S. Pat. No. 4,963,464, and Braat in U.S. Pat. No. 5,168,490.
A method for producing such a variable relief structure is required. Such a method exists in the art and is described by Wijdenes in U.S. Pat. No. 4,861,699. This method relies on using multiple layers of positive photoresist with interleaved optically absorbent separators. The idea is that by exposing the surface with a variable intensity light source all photoresist layers exposed above their respective photo-scission thresholds will be developed away leaving a pit whose depth is controlled by the intensity of the impinging light. This method suffers from the limitation that only certain, relatively deep steps may be produced due to the coverage properties of spin-cast photoresists. Typical spin casting produces photoresist layers in the 600 to 1700 nm range. More importantly, because the success of the method depends upon exposing only the desired number of upper layers past their photo-scission thresholds, a great degree of uniformity in photoresist sensitivity, photoresist light absorption and separator layer light absorption is required, both from batch to batch and across the surface of the coated disk. This will place an upper bound on the number of steps that can be produced.